WifiManager: Mark isScanAlwaysAvailable() deprecated

This API is useful for the apps to decide when they can issue a scan
request. Since WifiManager.startScan() is deprecated, this should be
too. There is no functionality change for now, just a warning
that this will go away along with startScan().

Bug: 74448981
Test: `make api-stubs-docs-update-current-api`
Change-Id: I5310dc53dae244d7a99319bbde111254e9097a14